Sustainable Fishmeal Alternatives: Impact of Partially Defatted Black Soldier Fly ( <i>Hermetia illucens</i> ) Meal on Growth and Health of Yellowtail Kingfish ( <i>Seriola lalandi</i> )

Reducing fishmeal (FM) in aquaculture diets is essential for improving sustainability and reducing reliance on marine resources. Black soldier fly (BSF; <i>Hermetia illucens</i>) larvae meal is a promising alternative protein source. This study evaluated the effects of replacing FM with BSF meal on the growth and health of juvenile yellowtail kingfish (YTK) (<i>Seriola lalandi</i>, initial weight ~22 g). Fish were reared in 24 tanks (three replicates per treatment) and fed for 33 days under controlled conditions. Eight diets were tested: a control (40% FM) and seven diets with BSF meal replacing FM at 25%, 50%, or 75%, with or without garlic and tuna hydrolysate additives. Fish fed 25% and 50% BSF diets showed growth and feed conversion comparable to the control, while 75% BSF significantly reduced growth due to decreased feed intake. Additives did not improve feed intake. Histological analysis indicated good gut health and nutrient absorption. Serum cholesterol decreased with BSF inclusion, and urea levels varied. No significant changes in gene expression were observed in the gut, liver, or brain. Microbiome analysis showed increased diversity and compositional shifts at higher BSF levels. These results support the use of BSF meal as a sustainable FM replacement at moderate inclusion levels, with further research needed to address palatability at higher levels.